This invention relates generally to surface wave, acousto-optic multi-product correlators, and more particularly to two, three, and four product correlators which features the interference between two product diffracted beams from a single four-channel surface acoustic (SAW) delay line. This product diffracted beam results from the interaction between the incoming light beam and a product acoustic "wave" generated by two counterpropagating surface acoustic waves.
Current digital and microwave technology has made possible spread-spectrum and other wideband communications and radar systems for antijam and low probability of intercept protection. These systems present unique problems for which acousto-optics may provide solutions. The relative ease in applying multiple transducers to surface acoustic wave delay lines allows novel architectures for such signal processing functions as correlation or convolution. Where large processing gain is required, integration in time rather than space permits time-bandwidth products in excess of 10.sup.6. Coherent, interferometric schemes provide both time (e.g., time-difference-of-arrival) and frequency information simultaneously.
As is known, the correlation function serves many useful purposes in the processing of radar and communication signals. Specifically, it is most useful when attempting to extract weak signals from a noisy environment, such as radar return signals, and in the process of synchronizing a spread spectrum communications system.
The gain of a signal processing system is essentially proportional to the time-bandwidth product thereof, where time refers to the integration time, and this product is a figure of merit of a processor. The interaction time, which may be different than the integration time, is the specific time window which is being simultaneously integrated, and in general, it is desirable to maximize the interaction time as well as the time-bandwidth product.
One type of correlator which has been developed in recent years is the surface wave acousto-optic type device, exemplified in U.S. Pat. Nos. 4,110,016 4,139,277 and 4,124,280, which are incorporated herein by reference. In such a device high frequency acoustic waves having envelopes corresponding to the signals to be correlated are propagated down piezoelectric crystals such as lithium niobate while a laser beam is directed across the crystals. The acoustic waves of the signals to be correlated diffract the coherent light, and upon suitable detection, the correlation function of the two signals is obtained. One limitation of the above described device is that it is often limited to use with signals having durations which are shorter than the interaction time of the device. The reason for this limitation is that the correlation integration is performed over a limited spatial variable, such as the length of the crystal delay line.
In U.S. Pat. No. 4,326,778, entitled "Acousto-optic Time Integrating Correlator", incorporated herein by reference, an acousto-optic time integrating correlator having a relatively high time bandwidth product as well as a relatively long interaction time is disclosed. While being an improvement, it is limited when it is used as a cross-correlation signal detector, by any difference between the reference carrier frequency and the input signal carrier frequency. For example, it has been calculated that for a 30 ms integration time, the device is limited to processing signals which are separated by less than 200 Hz. However, it is frequently necessary to cross-correlate signals of greater frequency separation, for example, in a radar system where the return radar signal is Doppler-shifted by reflection off of a moving target.
In U.S. Pat. No. 4,421,388 Dec. 20, 1983, entitled "Acousto-optic Time Integrating Frequency Scanning Correlator", incorporated herein by reference, an acousto-optic time integrating two-dimensional frequency scanning correlator for cross-correlating signals which are separated in frequency is disclosed. In that application, two coherent light beams which are derived from the same laser are fed across respective Bragg cells, one cell having the signal A(t)cos w.sub.A t propagating thereacross and the other cell having the signal B(t)cos w.sub.B t propagating thereacross. The respective output beams are compressed in the x direction and expanded in the y direction and are made incident on an acousto-optical correlator device having chirp signals counter-propagating thereacross. The optical output is fed to a time-integrating photodiode array which provides an output signal corresponding to the cross-correlation of A(t) and B(t). In a further embodiment, the two Bragg cells are replaced by a single Bragg cell and beams having different polarizations are fed thereacross. In a still further embodiment, only a single crystal is used which has the A(t) and B(t) signals, as well as the chirp signals, counter-propagating thereacross. However, the architecture of the two-beam devices is very difficult to implement optically. The two-beam architecture has the two beams going in to the Bragg cell with four times the Bragg angle between them to insure that the left incoming input laser beam interacts primarily with the surface acoustic wave (SAW) produced by the left hand transducer to give an output beam and likewise the right incoming laser beam primarily interacts with the SAW generated by the right hand transducer to obtain a second output beam, i.e., two output beams that are essentially colinear are obtained. It is then necessary to do some spatial filtering.
In U.S. Pat. No. 4,426,134 Jan. 17, 1984, entitled "Three & Four Product Surface-Wave Acousto-Optic Time Integrating Correlators," incorporated by reference herein, a method and device that provides two-dimensional three and four product correlated signals is disclosed. In that application a laser beam is split and shaped into first and second sheet beams. The first beam is directed to a first acousto-optic medium where it is doubly diffracted by first and second signals. The second beam is directed to a second acousto-optic medium which is spatially rotated 90.degree. relative to the first acousto-optic medium where the second sheet beam is either singly diffracted by a third signal or doubly diffracted by a third and a fourth signal. The diffracted sheet beams are shaped into square beams and combined, and the combined beam is directed to a photodiode array to be detected. However, the architecture of this two beam device is also very difficult to implement optically. The optical signals are subject to jitter resulting from the vibration of the individual optical elements. The combined signal presented to the photodiode array detector is of low intensity, resulting in an inefficient system.